WIP: bd-164 timestamp-only export deduplication (~80% complete)

Implemented content hash-based deduplication to skip exports when only
timestamps changed. Core logic complete, needs export_hashes table wiring.

Completed:
- Added computeIssueContentHash() excluding timestamps
- Created shouldSkipExport() logic
- Updated export loop to skip timestamp-only changes
- Added hash.go with content hashing
- Extended Storage interface

Remaining:
- Complete export_hashes table migration
- Add SetExportHash/GetExportHash to interface
- Revert content_hash from dirty_issues approach
- Wire up hash persistence in export
- Testing

See bd-164 notes for details.

Amp-Thread-ID: https://ampcode.com/threads/T-d70657d1-4433-4f7e-b10a-3fccf8bf17fb
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/sqlite.go

@@ -144,6 +144,7 @@ func migrateDirtyIssuesTable(db *sql.DB) error {
 			CREATE TABLE dirty_issues (
 				issue_id TEXT PRIMARY KEY,
 				marked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+				content_hash TEXT,
 				FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
 			);
 			CREATE INDEX idx_dirty_issues_marked_at ON dirty_issues(marked_at);
@@ -159,7 +160,25 @@ func migrateDirtyIssuesTable(db *sql.DB) error {
 		return fmt.Errorf("failed to check for dirty_issues table: %w", err)
 	}
 
-	// Table exists, no migration needed
+	// Table exists, check if content_hash column exists (migration for bd-164)
+	var hasContentHash bool
+	err = db.QueryRow(`
+		SELECT COUNT(*) > 0 FROM pragma_table_info('dirty_issues')
+		WHERE name = 'content_hash'
+	`).Scan(&hasContentHash)
+	
+	if err != nil {
+		return fmt.Errorf("failed to check for content_hash column: %w", err)
+	}
+	
+	if !hasContentHash {
+		// Add content_hash column to existing table
+		_, err = db.Exec(`ALTER TABLE dirty_issues ADD COLUMN content_hash TEXT`)
+		if err != nil {
+			return fmt.Errorf("failed to add content_hash column: %w", err)
+		}
+	}
+
 	return nil
 }